Now well known in the art of electronic particle counting and analyzing is apparatus marketed primarily under the registered United States trademark "COULTER COUNTER," owned by the assignee of this application. Such apparatus and portions thereof are disclosed in several U.S. Pat. Nos., for example: 2,656,508; 2,985,830; and 3,259,820 (each in class 324-71). A significantly important portion of a Coulter type of apparatus is a minute scanning aperture or sensing zone, through which particles pass and are detected at a rate often well in excess of 1000 per second. Because of the physical parameters of the scanning aperture, and because of particle concentration, coincidence of particles within the aperture occurs quite often. The effect is that one particle is detected and counted, instead of two. The coincidence of two particles in the scanning aperture is commonly referred to as primary coincidence. Furthermore, the simultaneous passage of two particles, both of which are too small to be included in a counter, creates an additive effect which produces a spurious count of a fictitious large particle. This is commonly referred to as secondary coincidence. Although such primary and secondary forms of coincidence are random in time and nonlinear, they follow a statistically ascertainable form from which curves, tables and formula are obtainable. It has also been found that the nonlinear characteristic of coincidence error is related to the repetition rate and duration of the pulses due to the detected particles. The present method and apparatus is directed towards the counting of particles having a narrow range of sizes, consequently secondary coincidence does not present a problem which must be considered.
Heretofore, the operator of an electronic particle counting and analyzing device such as is commercially available under the registered United States trademark "COULTER COUNTER" would obtain a particle count by passing a suspension of particles through the device. The operator would then refer to a coincidence correction chart which presented the proper error corrected count for a very large selection of counts produced by the device. Although the use of charts provides an accurate result, it is both time consuming and prohibits the fully automatic recording and processing of error correct counts. In addition, the accumulating count during analysis is uncorrected. Furthermore, different charts must be used with apertures of different sizes.
The use of analog, nonlinear meters and/or elements at the output stage of the "COULTER COUNTER" also has been accomplished with limited success, however, in many uses a direct ready digitized output is preferred. Heretofore it has been impossible to provide circuitry which compensates or accounts for the nonlinear characteristics of coincidence error while providing a direct reading digitized form of output which may be counted.